How to proper read ADC by ULP while ESP32 wake up - esp32

Question

I am going to continously measure ADC data with ULP. Periodically I wake up ESP32 to do some stuff. When I analyse data received from ADC I found that there is about 100 ms of time where ADC return 0xfff while wake up ESP32 core (ULP work continously)
I am using Olimex ESP32-DevKit-LiPo, PlatformIO with Hulp library.
With sample code (below), which measure ADC and after about 400 measurements it wakes ESP32, I always receive 4095 for about 100 ms and then ADC values return to measured values. In example code there is ULP period set to 4 ms, and there are 24 0xfff results.
Is it ESP32 bug (or feature) or what am I doing wrong (and how could it be fixed?)
below some results with format sample number --> ADC result,
0 --> 1891, 1 --> 1893, 2 --> 1890, 3 --> 1882, 4 --> 1881, 5 --> 1883,
. . . . . .
399 --> 1893, 400 --> 1887, 401 --> 1889, 402 --> 1891, 403 --> 1899,
404 --> 4095, 405 --> 4095, 406 --> 4095, 407 --> 4095, 408 --> 4095, 409 -->
4095, 410 --> 4095, 411 --> 4095, 412 --> 4095, 413 --> 4095,
414 --> 4095, 415 --> 4095, 416 --> 4095, 417 --> 4095, 418 -->
4095, 419 --> 4095, 420 --> 4095, 421 --> 4095, 422 --> 4095, 423
--> 4095, 424 --> 4095, 425 --> 4095, 426 --> 4095, 427 --> 4095, 428 --> 1904, 429 --> 1893, 430 --> 1890, 431 --> 1883, 432 -->
1879, 433 --> 1879, 434 --> 1877, 435 --> 1873, 436 --> 1881, 437
--> 1894, 438 --> 1904, 439 --> 1904, 440 --> 1903, 441 --> 1904, 442 --> 1905, 443 --> 1909, 444 --> 1910, 445 --> 1914,
#include "Arduino.h"
#include "hulp_arduino.h"
#define bufor_lnght 400
#define bufor_lnght_max bufor_lnght + 400
typedef union
{
uint16_t d_ADC[bufor_lnght + 200];
uint8_t d_ADC_8[(bufor_lnght + 200) * 2];
} dane_pom_X;
struct paczka_ws
{
uint16_t meas_id;
dane_pom_X dane;
};
paczka_ws dane_ADC_ws;
#define ULP_WAKEUP_INTERVAL_MS (4)
#define ADC_IN GPIO_NUM_34
RTC_DATA_ATTR struct
{
ulp_var_t ULP_meas_cntr;
ulp_var_t ULP_buf_RTC_to_RAM;
ulp_var_t ULP_ADC_result[bufor_lnght_max];
} ulp_vars;
void ulp_init()
{
enum
{
LBL_MEAS_ADC,
LBL_WAKEUP_TRIGGERED,
LBL_HALT,
};
const ulp_insn_t program[] = {
// Set a register to 0 for use with I_GET and I_PUT
I_MOVI(R3, 0),
I_GET(R0, R3, ulp_vars.ULP_meas_cntr),
I_SUBI(R0, R0, bufor_lnght_max),
M_BXF(LBL_MEAS_ADC),
I_MOVI(R0, bufor_lnght_max - 1),
I_PUT(R0, R3, ulp_vars.ULP_meas_cntr),
M_LABEL(LBL_MEAS_ADC),
I_ANALOG_READ(R0, ADC_IN),
I_GET(R1, R3, ulp_vars.ULP_meas_cntr),
I_PUT(R0, R1, ulp_vars.ULP_ADC_result),
I_ADDI(R1, R1, 1),
I_PUT(R1, R3, ulp_vars.ULP_meas_cntr),
I_GET(R0, R3, ulp_vars.ULP_meas_cntr),
I_SUBI(R0, R0, bufor_lnght + 2),
M_BXZ(LBL_WAKEUP_TRIGGERED),
M_BX(LBL_HALT),
M_LABEL(LBL_WAKEUP_TRIGGERED),
I_MOVI(R0, 1),
I_PUT(R0, R3, ulp_vars.ULP_buf_RTC_to_RAM),
M_WAKE_WHEN_READY(),
M_LABEL(LBL_HALT),
I_HALT(),
};
ESP_ERROR_CHECK(hulp_configure_analog_pin(ADC_IN, ADC_ATTEN_DB_11, ADC_WIDTH_BIT_12));
ESP_ERROR_CHECK(hulp_ulp_load(program, sizeof(program), 1000UL * ULP_WAKEUP_INTERVAL_MS, 0));
ESP_ERROR_CHECK(hulp_ulp_run(0));
}
void setup()
{
Serial.begin(115200);
if (hulp_is_deep_sleep_wakeup())
{
if (ulp_vars.ULP_buf_RTC_to_RAM.val == 1)
{
for (uint16_t i = 0; i < bufor_lnght + 100; i++)
{
dane_ADC_ws.dane.d_ADC[i] = ulp_vars.ULP_ADC_result[i].val;
}
ulp_vars.ULP_buf_RTC_to_RAM.val = 0;
for (uint16_t i = 0; i < bufor_lnght + 100; i++)
{
Serial.print(i);
Serial.print(" --> ");
Serial.print(dane_ADC_ws.dane.d_ADC[i]);
Serial.print(", ");
if (i % 6 == 5)
{
Serial.println();
}
}
}
}
else
{
ulp_init();
ulp_vars.ULP_meas_cntr.val = 0;
ulp_vars.ULP_buf_RTC_to_RAM.val = 0;
}
Serial.println("Sleeping...");
esp_sleep_enable_timer_wakeup(10000000);
esp_deep_sleep_disable_rom_logging();
esp_sleep_enable_ulp_wakeup();
esp_deep_sleep_start();
}
void loop()
{
// put your main code here, to run repeatedly:
}