Victor12 If you can made a video ( without share any data that cannot or you do not want share ) this can be helpful.
I am looking at the title of this discussion "A force-refresh fixes everything, temporarily" and this seems to be the same issue i have so the solution will be maybe look with the developers of Flarum when they are free.
Regarding issue on creating new post i don't have this, did you found other user that are experiencing issue on posting?
Today i am not too much well but maybe if you can share your website someone me or i don't know can look closer.
I tried Bazaar but discover in my case create only Flarum issues so i removed it an i use install from SSH composer at the moment.
You can try to do a near new Flarum install and try to migrate config.php , assets folder and maybe test if the issue persist. At the moment i have no idea on how to help. I had no posting issue on upgrading from Beta 7 to Beta 8