Narrow selection in an EnumList/Ref list

[SJKonthego]

My goal is to narrow down the options from an enum list based on the responses to a selection variable. So when I select "Air Conditioning" I want the only Action items to be related to Air Conditioning. I then want to be able to select one or many of the actions listed (this part is working fine).

The ACTION field is an ENUM list in the table PROBLEM with a REF base type looking up to table ACTIONS. Unfiltered, it looks like this first screenshot.

The ACTIONS table has columns for multiple systems (AC, Furnace, Refrigeration) and each of those columns contains either a 0 if it doesn't apply to that system or an appropriate letter (A for AC, F for Furnace, etc). Those columns are then concatenated into a single column ACTIONSYSTEM. If ACTIONSYSTEM contains an A you know that action applies to ACs if it contains an F you know it applies to Furnaces, etc.

My current (ineffective) approach is to put the following into the data validity section of the ACTION field (only working with ACs and Furnace in this expression to keep it simple):

IFS([System]="Air Conditioning", CONTAINS(ACTIONS[ACTIONSYSTEM],"A"),[System]="Furnace", CONTAINS(ACTIONS[ACTIONSYSTEM],"F"))

Here's what I get - see below - (1) the ACTION field has morphed into a different format even though it is still set to STACK; and (2) it appears to be filtering in some way, but it is returning options that don't make any sense (the three below do match ACs, but so do 3 others that aren't shown). What is odder is that I get the same three ACTION options if I select Furnace, and none of these apply to furnaces. 

My questions:

1. Is there a more elegant way to approach this entire problem?

2. If not, if this is a reasonable approach, can you assist with expression building? I've been reading, but clearly I'm not getting there.

3. Finally, any suggestions on how to get this back to a stack?

My apologies for how complicated this got. Thanks for guidance.