Ninja
I solved problem by deleting some templates overrides, so problem was template modification.. seems some functions were deprecated in J4 and cause such strange behavior. So it's not T4 bug in itself. Thanks you very much for support!
some details:
I used Ja Stark as example for my own template
it has such code in override for article layout (html/com_content/article/project.php)
it contain such part of code:
<?php if ((strtotime($this->item->publish_down) < strtotime(Factory::getDate())) && $this->item->publish_down != Factory::getDbo()->getNullDate()) : ?>
<span class="badge badge-warning"><?php echo Text::_('JEXPIRED'); ?></span>
<?php endif; ?>
this part of code cause that warning badge after upgrade to J4. Everything is ok after delete. Probably problem is with getDbo() function which is deprecated in J4. And of course I had no expired articles at all ("Finish Publishing" (publish_down) left blank for each article). BTW, I noticed this part of code is still used in JA Stark template current version 2.0.0, but maybe you did some corrections there?
Any suggestion how to change it?
ps
my site is still on WampServer so I can't send a link.