Workplace stress + ADT fatigue = burnout?

I was diagnosed 5 months ago, have been on ADT for 4 months, and am due to start radiotherapy in a couple of weeks.
I’m still working, taking time off only for appointments. My employer has been brilliant—going well beyond their duty under the Equality Act to provide reasonable adjustments, allowing me to work from home with full flexibility.
The thing is, I *want* to work. I’m a product engineer designing cabin parts for passenger cars. Work has been a great way to keep my mind off this pestilential disease; I genuinely enjoy solving technical challenges.
Unfortunately, over the last month or two, I’ve hit a human challenge I can’t solve: the program manager for my projects. On the surface, he’s affable and friendly. At first, I honestly worried he had early-onset dementia because of how much he forgets. I’d give him precise technical information, only for him to ask for it again days later.
On Thursday, after a long phone call, the penny dropped: he doesn’t have dementia at all. He’s just winging it. His sole objective is covering his backside. This contrasts starkly with my job as an engineer, which is to ensure we manufacture high-quality parts cost-effectively, within realistic timelines, and strictly adhering to the laws of physics.
During the call, he completely ignored a critical engineering instruction I’d given him. When I made it clear he could not proceed with his proposed plan, he told me I was "being silly," claimed it was just a "Plan B," blamed purchasing, sales, and the customer, and essentially BS-ed his way to the end of the call.
At that point, I hit a state of mind I almost never reach: *“F*** it.”* I hate seeing projects fail, but if a senior manager thinks his spin takes precedence over engineering reality, I felt powerlessly forced to let him burn it down.
After that call, I did something I’ve never done in my career. I set my status to offline, went to bed, and slept until 6:00 p.m. On Friday morning, despite having urgent tasks, I couldn't face it and stayed in bed all day.
Since then, I’ve been obsessively replaying that call, realizing how much energy this man spends spinning plates to avoid being unmasked as the root cause of weekly disasters.
More concerningly, my ability to connect with my family has temporarily shut down. When the grandchildren were over this evening—normally something I love—the noise overwhelmed me. I retreated upstairs. When I heard my wife coming up, I put a pillow over my face because I just couldn't engage.
I feel like I’ve hit a wall. In normal times—prior to prostate cancer and the grinding fatigue of ADT—I would have found a professional, pragmatic way to handle this guy. That phone call wouldn't have broken me. But it did.
**My question to the group:**
Is it normal for ADT fatigue and treatment stress to completely decimate your normal tolerance for workplace nonsense?
I’m seeing my GP on Monday to request being signed off, at least until radiotherapy is wrapped up. Because the core of my job usually protects my mental health, I’m hoping to return eventually on restricted duties—focusing strictly on creating useful things, completely divorced from office politics and arguing with managers.
Would appreciate hearing if others have experienced a similar sudden drop in their stress threshold while on treatment.