I know this is 2 months old but it caught my eye when replying to another post of yours today.
I noticed brightness changes when applying transitions and I think my workaround will also fix your brightness change for titles.I have just given it a go and was successful.
Note this workaround will not fix what you have already done, only what you do in the future.
Open up iMovie 6 and put the transition called Circle Closing anywhere convenient, just the one will do.
With this transition selected, ie still coloured blue, go to EDIT and then CUT.
Now put your title or titles in where you want and it/ they should not change brightness.
The reason that I have not noticed this before for Titles is because I always do the above workaround for Transitions.
You only have to put in and remove the circle closing transition the once for this to work ie as soon as you open your project and before doing any editing.
Once you have done it you can forget about it for this open project.
If you close this project down and reopen it then you must do the "circle closing' routine again on reopening as it does not remember from the last time it was open.
Have you had any brightness changes for transitions in the past?
There has been hardly any mention of this phenomena on these discussions so can only assume that it is not a big issue. A bit surprised though.
Don't forget this will only fix what you do from now ,not what you have already done.
Would be interested to know if this works for you.