Rails Admin POST requests cause H15 error on Heroku - heroku

Question

I have a rails app that is working fine on Heroku, but all the POST requests coming from Rails admin are hanging for 55 seconds before causing a H15 "Idle Connection" error.
I've read that it can come from:
ActionCable not pinging the server during 55 consecutive seconds but I'm not using real time features with Rails Admin and I removed my ActionCable code to test and the problem still occurs
The server tries to return a response to a different IP than the one that initiated the call once you've configured your DNS, but I'm not, I'm not using a custom domain name, I use the my website-herokuapp.com one.
Here's the log of a POST request from my app from the admin:
2021-01-24T19:09:36.245896+00:00 app[web.1]: I, [2021-01-24T19:09:36.245747 #4] INFO -- : source=rack-timeout id=21d5a60a-a82a-4cf7-bbe1-b0254f5823c7 wait=11ms timeout=15000ms state=ready
2021-01-24T19:09:36.246186+00:00 app[web.1]: D, [2021-01-24T19:09:36.246074 #4] DEBUG -- : source=rack-timeout id=21d5a60a-a82a-4cf7-bbe1-b0254f5823c7 wait=11ms timeout=15000ms service=0ms state=active
2021-01-24T19:09:36.246456+00:00 app[web.1]: I, [2021-01-24T19:09:36.246394 #4] INFO -- : [21d5a60a-a82a-4cf7-bbe1-b0254f5823c7] Started PUT "/admin/first_job/252990/edit" for 92.183.112.168 at 2021-01-24 19:09:36 +0000
2021-01-24T19:09:36.248487+00:00 app[web.1]: I, [2021-01-24T19:09:36.248393 #4] INFO -- : [21d5a60a-a82a-4cf7-bbe1-b0254f5823c7] Processing by RailsAdmin::MainController#edit as HTML
2021-01-24T19:09:36.248636+00:00 app[web.1]: I, [2021-01-24T19:09:36.248524 #4] INFO -- : [21d5a60a-a82a-4cf7-bbe1-b0254f5823c7] Parameters: {"utf8"=>"✓", "authenticity_token"=>"9QGOhotnG3BkLq6TbkXuXZgMVxJ+AinueV0oEUaVw5iaQ6DRJukISaCV4x+lcpXog9ExNYf9DB8/u/jufN6+Dg==", "first_job"=>{"company_name"=>"My company"}, "return_to"=>"https://mywebsite-staging.herokuapp.com/admin/first_job", "_save"=>"", "model_name"=>"first_job", "id"=>"252990"}
2021-01-24T19:09:36.285663+00:00 app[web.1]: D, [2021-01-24T19:09:36.285530 #4] DEBUG -- : [21d5a60a-a82a-4cf7-bbe1-b0254f5823c7] User Load (1.5ms) SELECT "users".* FROM "users" WHERE "users"."id" = $1 ORDER BY "users"."id" ASC LIMIT $2 [["id", 307509], ["LIMIT", 1]]
2021-01-24T19:09:36.307192+00:00 app[web.1]: D, [2021-01-24T19:09:36.307039 #4] DEBUG -- : [21d5a60a-a82a-4cf7-bbe1-b0254f5823c7] FirstJob Load (2.0ms) SELECT "first_jobs".* FROM "first_jobs" WHERE "first_jobs"."id" = $1 ORDER BY "first_jobs"."id" ASC LIMIT $2 [["id", 252990], ["LIMIT", 1]]
2021-01-24T19:09:36.312261+00:00 app[web.1]: D, [2021-01-24T19:09:36.312087 #4] DEBUG -- : [21d5a60a-a82a-4cf7-bbe1-b0254f5823c7] (1.0ms) BEGIN
2021-01-24T19:09:36.317945+00:00 app[web.1]: D, [2021-01-24T19:09:36.317805 #4] DEBUG -- : [21d5a60a-a82a-4cf7-bbe1-b0254f5823c7] Company Load (1.8ms) SELECT "companies".* FROM "companies" WHERE "companies"."id" = $1 LIMIT $2 [["id", 26218], ["LIMIT", 1]]
2021-01-24T19:09:36.322239+00:00 app[web.1]: D, [2021-01-24T19:09:36.322101 #4] DEBUG -- : [21d5a60a-a82a-4cf7-bbe1-b0254f5823c7] (1.1ms) COMMIT
2021-01-24T19:09:36.323441+00:00 app[web.1]: I, [2021-01-24T19:09:36.323351 #4] INFO -- : [21d5a60a-a82a-4cf7-bbe1-b0254f5823c7] Redirected to https://mywebsite-staging.herokuapp.com/admin/first_job
2021-01-24T19:09:36.324756+00:00 app[web.1]: I, [2021-01-24T19:09:36.324674 #4] INFO -- : Completed 302 Found in 76ms (ActiveRecord: 19.6ms)
2021-01-24T19:09:36.324858+00:00 app[web.1]: I, [2021-01-24T19:09:36.324425 #4] INFO -- : source=rack-timeout id=21d5a60a-a82a-4cf7-bbe1-b0254f5823c7 wait=11ms timeout=15000ms service=79ms state=completed
2021-01-24T19:10:31.518929+00:00 heroku[router]: at=error code=H15 desc="Idle connection" method=POST path="/admin/first_job/252990/edit" host=mywebsite-staging.herokuapp.com request_id=21d5a60a-a82a-4cf7-bbe1-b0254f5823c7 fwd="92.183.112.168" dyno=web.1 connect=0ms service=55282ms status=503 bytes= protocol=https
Oh and of course, everything is working fine on my local.
Do someone has an idea?