I implemented this in fresh Magento 2.2.3 instalation: https://magenticians.com/add-custom-tab-magento-2/
After I make a change in code, like temporarily commented out:
<!-- <block class="Magenticians\Mymodule\Block\Mymodule" name="mymodule" template="Magenticians_Mymodule::mymodule.phtml" /> -->
and ran these in terminal: php bin/magento setup:upgrade php bin/magento cache:clean
...new tab is still there (changes didn't applied)...
I also tried activating developer mode: php bin/magento deploy:mode:set developer
Changes don't apply...
Please help.