[Buffalo4]

How does the ATF look on the dipstick? Clear, not burnt smelling etc? Is it at the Full mark or even on the dipstick? A super low fluid level will cause immediate slipping, similar to what you are experiencing.
Something very strange about it happening just after a fluid drain and fill OR, was it 3-4 drain and fills?
I'm losing track.
Please post back what ATF your mechanic installed and what he did. (Single drain and fill, Multiple, etc.)
To me it sounds exactly how a low ATF level would react.
Buffalo4

 

[Buffalo4]

Given that it was running perfectly coming into service and ran like crap coming out of service (and that the old fluid look just like new coming out), I don't think it's a pressure switch problem but more of the fluid problem.

Thank you odyfamily.

I also ordered Lubeguard red to try out.

Please change the ATF before you add Lubegard Red or anything else.
What is the fluid level on the dipstick? Color, smell?
Drive it as little as possible until you change that ATF fluid (3-4) drain and fills with a little driving time in between each.
If all then is changed with no big positive results, you have other problems. Could a switch have become disconnected?
Buffalo4

 

[Buffalo4]

The ATF looks good on the stick, good level checked while hot.
It is a single drain-fill
He changed ATF for me before (I let him change it on Honda schedule for documentation, I change the ATF myself for every A1 as a "silent" service just for my piece of mind). Everything was OK the last time using the same fluid as this time.

One thing I didn't mention was that during the fill, he spilled some outside and made a little mess so he sprayed some cleaner and water to clean the front of the engine. I do see some fluid remain on the electrical connectors. I don't know if this could cause problem. I was more worried about getting the VCMMuzzle2 connector wet because I'm not sure if it is made to the same standard as the rest of the connectors. I think new engine can take a little water hosing without problem but I never feel comfortable doing that myself.

Sounds like you are getting closer to solving the problem.
Even using the wrong ATF with only a single drain and fill will not cause your problems.
It is most likely an electrical problem due to the spill and/or attempted cleanup.
Sure hope that is all it is.
Buffalo4

 

[Buffalo4]

Makes me wonder if the mechanic might have added 'his' secret' ATF additive?
Or, could it have been an electrical connection in one of the solenoid connectors that just happened to make good contact at the same time the ATF was changed?
One drain and fill causing that first problem almost immediately and the last drain and fill fixing that problem almost immediately . ???
Lots of 'IFs'
Buffalo4
PS: Very happy to hear that it is working well now. If that 'other' ATF is still suspect, I would surely do 3 more drain and fills.

 

[Buffalo4]

Try disconnecting the battery for a few hours.
Try driving again.

Instead of waiting a few hours, just touch the cables together for a second or two AFTER you disconnect the Neg from the battery, of course, Otherwise, a possible explosion and/or bad acid burn.
This will drain the capacitors and that is the same thing that is what is done by leaving it disconnected for a long time.
Leaving it disconnected is somewhat safer as long as you disconnect the Neg.
Buffalo4