I am seeing almost the same issue as described by Riquez in this post.
I’m using Libre Office Writer 7.5.22 on elementary OS 5.1.7 Hera (Ubuntu 18.04.6 LTS)
I have checked the text language in the status bar. This is correctly set to English (UK). I have tried changing this to English (USA) but it makes no difference.
I have checked the settings in the Default Style and also in Language Settings under Tools>Options. I’ve verified the correct dictionary is installed and check marks are showing as required.
I tried restarting in safe mode but this made no difference.
This is where it starts to get strange. I have a short story, written by my father, that I am proof-reading. The spell check does not work on this document but if I type a few random phrases in a new document it works fine. So, I decided to copy and paste the text from the source document to a new one but still no spell check.
I then tried pasting just a few sentences and testing them. I found this worked for a while, but then I encountered a failure. I gradually narrowed the issue down to on word - time.
If I have the word ‘tim’ and run the spell check, it correctly prompts me to change it to ‘time’. However, after doing this the spell checker no longer works.
I then tried creating a brand new document, with the same result. ‘tim’ works but ‘time’ doesn’t!
Following this, I updated to the latest Libre Office version (I was previously on 6.4.something) but it made no difference.
I am completely flummoxed and would appreciate any suggestions…