You're in a union, talk to your union rep, bring documentation of the incidents and see what they think and if they can help. If they agree and help, great.

If they can't help, then anytime anyone outside your management chain asks you to do something, you can say something to the effect of I'd love to help, can you just ask my manager for permission to prioritize your task over their normal priorities for me. If your manager/management chain asks you to do something, make sure you tell them what won't get done properly or timely if you comply with their task "If I do XYZ tasks, then I won't have time to finish ABC priority today", if they're ok it, then you document it and suck it up.

The keys here are: always act as if you're willing and happy to help, you only do work authorized by the people who can give you work, the people who give you work are the bad guys if they say no and they become aware of all the extra requests of your time, don't overload your trying to carry your own work and someone else's, document as much as possible in case someone in your management chain has an issue with you not having done something that a manager agreed to.

That's what's worked for me in the corporate world at least, not sure what your environment is, so YMMV.