ffmpeg concat of multiple files audio out of sync - audio

Question

I am trying to concatenate multiple files together using ffmpeg concatenate demuxer. However final video is out of sync.
First parts are coming from static image which are converted to 5-seconds video in the following way:
ffmpeg -r 30 -i 1.png -vf loop=loop=150:size=1:start=0 -pix_fmt yuv420p -c:v libx264 -preset superfast -tune stillimage loop.mp4
Then I add silent audio stream:
ffmpeg -i loop.mp4 -f lavfi -i anullsrc -map 0:v -map 1:a -ar 44100 -ac 2 -c:v copy -c:a aac -shortest silent.mp4
ffrobe for the silent.mp4:
ffprobe -v quiet -show_entries stream=start_time,duration silent.mp4
[STREAM]
start_time=0.000000
duration=5.733333
[/STREAM]
[STREAM]
start_time=0.000000
duration=5.665011
[/STREAM]
Which already shows that audio and video streams have different duration.
Then I prepare concat input file, last line is the video with same dimension and framerate, also it has existing audio stream (44.1kHz, stereo)
file silent.mp4
file silent.mp4
... (multiple lines, say 10)
file silent.mp4
file video.mp4
To make sure that inputs have same parameters:
ffmpeg -i silent.mp4 -i video.mp4
ffmpeg version 4.2.2 Copyright (c) 2000-2019 the FFmpeg developers
Input #0, mov,mp4,m4a,3gp,3g2,mj2, from 'silent.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf58.29.100
Duration: 00:00:05.03, start: 0.000000, bitrate: 32 kb/s
Stream #0:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080 [SAR 1:1 DAR 16:9], 19 kb/s, 30 fps, 30 tbr, 15360 tbn, 60 tbc (default)
Metadata:
handler_name : VideoHandler
Stream #0:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 2 kb/s (default)
Metadata:
handler_name : SoundHandler
Input #1, mov,mp4,m4a,3gp,3g2,mj2, from 'video.mp4':
Metadata:
major_brand : isom
minor_version : 512
compatible_brands: isomiso2avc1mp41
encoder : Lavf58.29.100
Duration: 00:00:43.03, start: 0.000000, bitrate: 1622 kb/s
Stream #1:0(und): Video: h264 (High) (avc1 / 0x31637661), yuv420p, 1920x1080, 1484 kb/s, 30 fps, 30 tbr, 15360 tbn, 60 tbc (default)
Metadata:
handler_name : VideoHandler
Stream #1:1(und): Audio: aac (LC) (mp4a / 0x6134706D), 44100 Hz, stereo, fltp, 128 kb/s (default)
Metadata:
handler_name : SoundHandler
Now, after concatenation final video is out of sync with audio (audio stream of the last part starts before static part is finished)
ffmpeg -f concat -i concat.txt -c copy result.mp4 (no warnings)
I have tried to pad audio stream with silence in both loop.mp4 and video.mp4, but it did not help - it randomly modifies duration and start time of both video and audio streams and audio is again out of sync after concat.
Also, I am not able to increase the duration of static part (so that number of entries in concat.txt can be decreased) because each static part can be different - this is just an example.