I use a fair number of memo fields because of working in the healthcare field; we're big on Progress Notes and such. Because of this I'm always looking out for anything on the topic. The following is a distillation of things I've picked up over the past few years. To give you an idea, I keep it in a file titled MemoFieldAngst!
Crosstab queries, Summary queries, Union queries, and Queries that use Distinct or DistinctRow will all truncate a memo field to 255 characters so Access can perform the required functionality of eliminating duplicates.
Also, if you have specified a format in the field's Format property, this will often truncate the data as well.
If Unique Value Property is set to Yes, Access has to compare the values and therefore Memo Field values are truncated.
The bottom line is, if you use memo fields keep it simple! No fancy formatting and no fancy data manipulation!