"Sync done" yet nothing sync'ed on Android 10 ?

[hardtopnet]

Steps to reproduce

1. Use Google Pixel 3a with Android 10 / Android Q ROM
2. Install Nextcloud SMS from F-Droid forcing installation of incompatible versions (tested 2.0.4 and 2.0.5)
3. Execute, enter credentials and start sync
4. The application displays "Sync done" or something along those lines (my installed application is in French) after a couple of seconds

Expected behaviour

ocsms application in Nextcloud instance should display new sync'ed messages

Actual behaviour

messages don't show in ocsms application

Server configuration

Synology DS218+

Nextcloud version:
14.0.4

PHP version:
7.0.33

Webserver:
nginx on Synology WebStation

HTTPS:
enabled

Client configuration

Android version:
2.0.4 and 2.0.5 from F-Droid store

Phone:
Google Pixel 3a with Android 10 ROM

Nextcloud SMS app version:
2.1.6

Logs

(could you tell me how to get those logs ? I don't see anything relevant in /var/log nor in /nextcloud/data/nextcloud.log)

Screenshots

additional info

The server-side configuration should be right because it worked as expected with my previous phone (Xiaomi Redmi 5 plus with Android 9)

[g6094199]

4 me the same behavior on Lineage 17, Nextcloud 20 and 2.0.5