Bootstrap is concerned as a powerful front-end framework that brings a faster and easier way in developing websites. In particular, as aiming at creating responsive design, we can't look further than Bootstrap.
Bootstrap 4 is sooner or later
For your information, as Joomla's roadmap stated, Joomla 4 will be released soon in the end of 2019. According to that, Joomla 4 will come with the integration of the latest version of Bootstrap 4, instead of Bootstrap 2.
Therefore, 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. In fact, some users didn't like it, but you have an option also.
How to back to Bootstrap 3
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
We received feedback about the styling issue with default Bootstrap 4 after updating TZ Portfolio+ 2.3.2. That's why version 2.3.3 has been released so soon to address this issue.
Step 2: Go to Admin > System > Global Configuration > TZ Portfolio+ > tab Advanced Options > Bootstrap version > Switch back to "Legacy"
I hope this guide would be helpful for your case. If it's not, feel free to give us your feedback as soon as possible. We're willing to help you to figure it out.