Earlier quoted context omitted.
That's been my experience as well. My workaround for part of that is using many SSIDs. A. The SSID that covers both bands, in all areas B. Two more SSIDs, one for each band -- again, used in all areas C. Another SSID just for the AP in the garage (which also has A and B SSIDs) It has some advantages: I like being able to set a portable device to SSID A. These things usually figure it out well-enough while moving arou…
This also impacts maximum throughout / performance. You want to try at limit to 4 BSSIDs per AP, tops.
What are the nuts-and-bolts reasons that would make 5 perform worse?