Assuming you are using the Dynamic Container in your configuration, the current code implementation in pzpega_ui_dynamiccontainer.js tells me that what you are experiencing is expected. If your title character length is more than 16, it is chopped off after the 13th character and the rest of the string is substituted by "..."
The same logic exists in 7.1.8.
You could possibly open a SR to see if any flexibility around this configuration can be provided at all.
I can give you a little more detail and insight into this limitation.
If you are using a static set of tabs, (no Dynamic Container), then you should be able to leverage the stretch option on the tab layout group configuration.
This will allow you to have a more lengthy tab title.
But if our assumptions are true, (that you are using a Dynamic Container) then no need to open an SR. This stretch option will disappear when Dynamic Containers are being used and the title limit is 16 characters. This was a conscious design decision made by our UI product owners in order to better support readability/fitment when many tabs are being opened. As Sunny showed above you can see the logic which we have coded for this.
I have submitted an enhancement request in the past to make this limit configurable FDBK-9948. So you can reference this ID if you want to engage your Account Executive and make a push to have this feature enhanced in a future release.
Also, if it is of any help to you, you can consider adding a simple tooltip of your full tab name to better enable your users' understanding of what the tab represents.