Bootstrap is concerned as a powerful front-end framework that brings a faster and easier way in developing websites. For your information, Joomla 4 will be released soon in the end of 2019, and you can easily check its roadmap HERE. According to that, Joomla 4 will come with an integration of the latest version of Bootstrap 4, instead of Bootstrap 3.
In fact, we're on the way to move TZ Portfolio+ from Bootstrap 3 to Bootstrap 4. Since version 2.3.1, Bootstrap 4 has been set as a default configuration of Bootstrap version, which causes a little change in the style of your portfolio pages. Some users didn't like it, but you have an option also.
If you prefer to stay with Bootstrap 3, you can easily switch the bootstrap version back. Here below are simple steps that may help you out:
Step 1: Update TZ Portfolio+ 2.3.3
Step 2: Go to Admin > System > Global Configuration > TZ Portfolio+ > tab Advanced Options > Bootstrap version > Switch back to "Legacy"
If this guide doesn't take effect, feel free to give us your feedback as soon as possible.
TZ Portfolio support team