Confirmed. And it's likely related to the Search Tools implementation

Just found out that when I change

$clientId = $app->getUserState('com_modules.modules.filter.client_id', 0);

to

$clientId = $app->getUserState('com_modules.modules.filter.client_id', 1);

in models/select.php::populateState(...)

I only see backend modules.

I think it's related to the change in #6958.
There, the userstate name of the client_id changed from $this->context . '.filter.client_id' to $this->context . '.client_id'

Changing the line you mentioned to $clientId = $app->getUserState('com_modules.modules.client_id', 1); seems to fix it.

From code reviiew perspective don't consider that the riight fix.

As @Bakual mentioned there is a change in name of variable. So, instead of changing the default value when variable not found, make sure you look for the correct varable!

That would make sure the setting is respected (as intended) instead of ignoring it and only show an arbitrary choice (frontend or backend, depending on 0 or 1). Obviously, when the filter is not set showing frontend is fine with me. Don't see a reason to change that!

So, proper fix would be the removal of .filter
Or even better keep the .filter and change the code that is missing the .filter. That would be much more consistent with the rest of the filters

[Comment removed. Too complicated for me.]

See PR #8626.
The simple way (that I understand).