This is status-bydesign, see the main meta post here:
Yep, this is definitely by design. Block-level elements like blockquotes, code blocks or spoilers won't work in Markdown tables. – Ham Vocke♦
You could probably come up with some weird and funky workaround to still include spoilers in the table but it would most likely have more cons than pros for it.
Really for this particular case you're going to need to ask yourself:
- does the answer really need to be moved to table markdown?
- and do the spoilers really need to stay hidden behind the spoiler markdown?
Personally I'm not sure it really needs to move to table markdown in the first place so you might just be better leaving that answer as is for now.