I have "Development Mode" enabled before this feedback.
You are using "T4 framework v 1.1.5" and I am currently testing "T4 Template Blank 1.1.0". It seems that Development Mode may not be working properly in T4 Template Blank 1.1.0.
I am currently testing to decide whether to adopt T4 in the future. So far, we have adopted Helix. We are currently considering future frameworks for Joomla 4.
For security reasons, we are currently unable to provide our FTP information or anything else for investigation for now.
P.S.
I initially started testing T4 on Joomla 4. However, due to an issue I'm currently feeding back on, I'm testing with a fresh install of Joomla 3.9.