Btw, i think you should separate hashtag with comma instead of #. :)

If you want to get #foss #linux #mint
You should write words separated by comma in the hashtag field.

Example :
foss,linux,mint

And piefed will separate them.

Lastly point. If to want to use then efficiently, they must be common, popular. If you use : #digitalcommongood they may not appear in mastodon's user timeline because no one beside you use those hashtags. #foss are more common and that's up to you to find the correct one :)