On its own it's not necessarily an issue, but I'd consider it a warning sign. The times in my life that I've obsessively played games like this have been times when my emotional health was suffering. I felt overwhelmed by life and the world, and games like this gave me a synthetic feeling of progress and accomplishment, gave me something extremely simple to do that I couldn't fail at. Games like this were a symptom of my problems at the time, not the cause, and when my life got more stable, I lost interest in playing them.
If they're playing in moderation, just to pass the time during otherwise-boring events, probably not an issue. If they're pretty much always playing, or if it's intruding on their life, or if they're not otherwise engaging with the world, consider worrying about their emotional health.
Thank you! I noticed too I played much more games when in low mood seasons. Especially Minecraft (inb4: yes, I'm 40yo but play Minecraft) has a lot of small quests I could achieve. Also, the old Diablo I play on my phone (DevilutionX) allows me to kill time (and beasts) but I find it repeatable, yet still satisfying.
If they're playing in moderation, just to pass the time during otherwise-boring events, probably not an issue. If they're pretty much always playing, or if it's intruding on their life, or if they're not otherwise engaging with the world, consider worrying about their emotional health.