Audrius, yes, they are Thunderbird databases (and other files) synced across three devices. But I have been using this strategy successfully, thanks to syncthing, for two years without any issues. One instance is always closed and synced before another device opens the application and reads the synced files.
Your comment does not explain why reverting to 14.18 syncs files successfully. The problem I have described occurs only with 14.19.
Try 0.14.20 (upgrade button in the UI). Then, if that does not work, try setting weakHashThresholdPct in Advanced > Folder to 101 and see if that affects things.