Map Stash Tabs were originally introduced at the end of 2017, allowing players to store up to 72 of every type of Map from a specific series. Issues arose once a 5th series of maps was added alongside content update 3.5.0 (Betrayal).
As stated by Bex, Community Director at Grinding Gear Games, “when the latest league migrates to Standard, their Map Stash Tab is set to hold maps from the last series, so new maps can’t be inserted”. This would force players to manually flush out their entire stash tab to add the new maps (and then start looking for places to store the old ones).
A button which updates the series of the Map Stash Tab to the most recent one and bulk-converts all of the maps in it into equivalent ones at the right tiers from the new series.
Players wanting to keep and collect sets of maps from old series can then simply temporarily move the maps in question and click the button, after which they plop them back in.
The fix is expected before the end of the Synthesis league (currently set for 4th June for PC, 7th for consoles).