Weird de-indexing of my website

[Shwepps]

Starting on May 21 my site started slowly being de-indexed in google. That's what my gsc shows:

The number of HTTPS urls also goes down (and it also started on May 21):

The same goes for mobile usability pages (although it has started on May 19):

Before that, on May 10 I started pages validation (not all new pages didn't want to get into index), it ended yesterday (June 10) and that's what I've got:

I have no idea wtf is going on. If it was a penalty, the fall would be visible, but the number of keywords and impressions are going up. I haven't changed anything regarding https, page speed, etc.

Do you have any idea what can be the reason for this?

 

[IM Dude]

Algo penalty don't need any notification to be given to you. Could be anything but low quality content is the latest one.

 

[Shwepps]

Algo penalty don't need any notification to be given to you. Could be anything but low quality content is the latest one.

Does low quality content reduces number of mobile usability and https pages?
Also:

the number of keywords and impressions are going up

Contradiction it seems to me.

 

[IM Dude]

Does low quality content reduces number of mobile usability and https pages?

you wanna use one one protocol https
not http
or www
mobile usablity etc are all crap reasons google with give, it just want to de-index that's all.

 

[Shwepps]

you wanna use one one protocol https

I'm using https only.