Hello Gerard,
A link-to-table field and a look-up fields can contain multiple items which makes them arrays or lists. The function isblank() on the other hand expects a simple value like text, number, date,… This explains why it works on single text fields and not on link-to-table, lookup or rollup fields.
So, you first need to convert the list to a text. This can be done with the join() function. The result is a string that is empty when there are no elements in the link-to-table or lookup field. This will respond correctly on the isblank function.
A small (useless) demo.
I check if the field Organization (link-to-table field) has a value. If this is true, I show the content of the field Name. Otherwise, I show the content of the field Note. The formula is:
if(isblank(join(field('Organization'),'')),field('Notes'),field('Name'))
Hope this solves your problem.
Regards
Frederik