Break logo inheritance in subsites - Get Initial Letters in subsite logos

Hello folks,

Today I have experienced some particular issue regarding our subsite's logos. Yesterday all my subsites had their logo as the initials of each subsite, which is fine for us, but today I noticed that all logos changed to our root site collection logo, which is also the initial letter of this site.

When I tried to change the logo back in - Site Settings -->"Look and Feel"--> "Title, description, and logo" - it saves, but right after it reverse the logo to the main site where all subsites are sitting under. This issue is extended to theme of each subsite, but surprisingly a few subsites still kept their theme, most of them not.

I also noticed that the main site logo was modified this morning at 6am by the "System Account", which also I don't know that much about: 

Has anybody faced the same issue before? I just wanted my ordinary initial letter's logo in all my subsites back 😞 

(and of course change the theme for these subsites).

***Just a reminder that it isn't a hub or communication site, all of them are team sites.

Cheers!