Defense Sub

Can anyone tell me why they use defense sub when i have jennings at short and they keep bringing in miranda? I have no sub replacement listed on team strategy.

Other than Miranda having a better defensive rating than Jennings I wouldn't know

I have struggled with that same problem. I even tried to put my SS (I think it was Dahlen) as the defensive replacement and he still kept getting replaced. One of those HAL-isms that we seem stuck having to live with.

I've never tracked it to see if it is consistent, but sometimes it seems that if it starts making a defensive sub it makes other unrelated substitutions. But if it didn't make the first it won't make the second. Probably not clear, let me give an example. I was platooning CFs. The one against RH was a 1 the one vs LH was a 3. So I had the 1 CF listed as a Defensive replacement in CF. In games where opponent started a LH my 3 CF would start and the 1 would sometimes sub. Obviously when the 1 started he usually finished the game.
So my catcher was also a poorly rated defensive rated player , but my backup was a cheap good fielder.
So on days when it replaced my CF it often changed catchers, but not if didn't change CFs. I had no defensive replacement set for the catcher.
I don't see that there should be a relationship between the two. Maybe once HAL starts subbing he can't stop himself.

Again, I didn't really track it about when it happened, game condition and how often. It just seemed like something that was happening. So maybe it is just one of those things you notice when it happens so my brain is putting a pattern to it where there isn't really one.

Yes, HAL sometimes puts in defensive subs on his own. It seems to affect some players more than others and the defensive difference doesn't need to be that great.
I had a team that kept bringing in Ed Brinkmann for Arky Vaughan.
Yes, 2e16 is better than 2e27 but not that significant- similar to the improvement from Dahlen or Jennings to Miranda.
I could understand bringing in a defensive wizard for a 3 or 4 fielder.

Another one is when you have a poor fielder set to remove for defensive replacement.
Then he comes in because of an injury to the starter.
Next thing you know, it's the 8th inning and he gets pulled but there are no other players who can field the position.
So HAL starts to move the third baseman to catcher, left field to shortstop and all sorts of weird shifts.
Now you have 5's all around.

You should see it in the 2020i set!!! its rampant, atleast for the one, and only team I have in that set. I only have 1 def replacement set, I start Ty France at 2nd, hes a 4e17 I think, grabbed me a .50 backup thats a 2e5 at 2nd... my team is fairly bad, but the 8 wins I have, my def replacement was only brought in twice! HOWEVER... There are many instances where 3-5 fielders have been swapped out to start an inning, sometimes, as early as the 6th! I kinda just sit back and think , well... HAL knows better than I do (I guess)

If you set ANY player to "Remove For Defensive Sub" on the hitter settings page, it will wreek havoc with your lineup, HAL starts doing all kinds of crazy stuff.

Only use the team strategy page to set defensive subs and don't bother trying double switches either, HAL's too stupid to do that either.

I only did double switches once and it worked out. I had a team where I used bad fielder at 3b against LHs and A-Rod who plays 3b and SS playing SS. Then had def replacement that moved A-ROD to 3rd and a good fielding SS into the game.
HAL always seemed to do it right. I was careful to follow the instructions in the HELP screen carefully to put the players in the right slots and in the right order. I don't remember the rules but if you go into the help in the Team Strategy page it lays it out. But it says double switch only works if you follow those rules.