I am using the 30w power adapter currently sold (not the original 29w). Also, second generation Apple HDMI adapter. If I remember correctly, the second one allows 60MHz 4K while the first one 30w only. Maybe the original sold adapter works…
What model number does your adapter say? Really small print on the adapter.
Interesting, I talked with Apple and they told me that the adapter will not work with power attached on the 2015 MacBook when I inquired.
It works with yours?!?
The adapter I am currently using is the second gen A2119. Originally I had the first gen adapter however I thought I was going to get a 4k monitor so I bought the 2nd gen about a year ago to future proof myself for 4k output on my 2017 MacBook.
I also have a base 2015 MacBook which also works no problem with both the first gen and second gen adapter with power and HDMI output to a monitor.
I bought the first gen Apple adapter from the Apple Store on advice from the staff at the store when I asked how I could output to HDMI and charge from the single port.
I am finding it hard to believe both my 2015 and 2017 can work with the adapter and power supply without the issues you describe. I would visit the Apple Store again and speak to someone else if you can.