Hi there,
The third URL you are referencing, is actually indexed:
https://dmitrii-regexseo.tinytake.com/tt/NDY4NDY4N18xNDgzNjgzMA
As for "crawled, not indexed" - in most cases it happens because of one and only reason - Google is seeing your page as thin content, not worth being indexed. Typically it happens on bigger sites with a lot of similar pages. In your case, you got many courses, with exactly same structure. So, if the content is not completely different, then Google might deem it not worthy.
As for the bug you referenced - did your URLs drop off the index exactly at the time when this issue has been discovered? (aka within the last week?).
Do you have any cannibalization happening?
To me it looks like that's the case. If I do this search: "site:https://www.ihasco.co.uk/ Sexual Harassment Training course"
There are many pages that are indexed and are ranking: https://dmitrii-regexseo.tinytake.com/tt/NDY4NDcwN18xNDgzNjg4Mg
So, basically, you have pages that are more authoritative with similar content. Therefore your courses pages are dropping as thin content.
I would recommend doing some internal linking optimization to tell Google what is actually important. Look in GSC for internal links metrics.
Hope this helps.