TLDR: Meet all of your commitments and your customers will come to trust you with more. Burn them once on timeline and they'll take it upon themselves to prevent it from happening again.
Yes, I have noticed this, and have done it myself. Most of the shops I've worked with over my career have frankly sucked at meeting their promised delivery dates, so this doesn't surprise me.
If I pay an unusually large expedite fee so a shop can drop someone else’s work and do mine, that's not my problem other than finding the money. If I pay the same expedite fee so that I can delay my co-worker's more important project over mine that's just bad business. If I tell the shop "delay the delivery date on project X so that you can get this new project Y done faster" they'll work with me, but be rightfully unhappy about the churn.
One of the projects I'm on right now I've split up across 3 shops for this very reason. It's a batch of a few dozen each of 3 different parts.
I have a great relationship with shop A, and the owner is very honest with me. He straight up told me that he could take on at most one of the 3 parts in my assembly and any more he would just be late on everything, so I only gave him the one part.
Shop B is near capacity on a different project of mine. I know this because they can't deliver parts for that project in the volume I want. They're also pretty honest with me and have told me they can't handle much unless I'm willing to accept a slight slow down on the other project, which I'm not. Result is that I only gave them one of the remaining 2 parts.
Shop C is pretty new to me. Not local and I don't really know them. They're a bit of a wildcard, and have also expressed that they're not confident they will hit the delivery time I requested, so they get the remaining part.
Turns out there aren't a ton of people in my network who want to do precision turned bits in under 1 week lead time (delivered, from first drawing). Not a surprise, it isn't really a reasonable ask and we're thankful for the shops we work with for being able to do this every now and then.
Here's the catch, most of the shops I've worked with over my career just say yes, and then promptly miss on both. When the delivery date passes and I ask where my parts are they proceed to blame me, because I've asked for so many things all at once! Part of it has been my fault, sometimes they've told me they can't hit it, my boss has told me to call back and demand better, and then they say yes and make up a date we both know they'll miss. In the end they're pissed because I'm calling about late parts, my boss is pissed because my project is late, and I'm pissed because I knew this was going to happen and my a**hole boss forced me into it.....again. Fortunately I no longer work for that boss and I don't have to buy parts from those shops, but you can see how this all happens.