Can text fields be not loaded for join memory purposes in MySQL?
Maybe.  That is, there is no setting to achieve such; however, rewriting the query can achieve such.
Please show us the query that is causing such.  Meanwhile, here are some possible examples:

SELECT * ... loads all the text columns.  If you don't need all the columns, spell out what you need instead of saying *.

SELECT big_text FROM t WHERE ... ORDER BY ... LIMIT 10 -- In some cases, the optimizer can focus on the 10 rows desired, and load big_text for only those.  For other cases, it loads big_text for 'all' the rows before discovering which 10 you needed.  This can usually be worked around.  But I need your SQL to talk you through it.

JOIN is another case where the previous item may or may not show its ugly head.