What would cause this youtube embed string to stop working?

I save youtube links in a datastore for viewing in batches according to topic. I basically save the videoid with a caption note and the length data.

I used php to construct a custom viewer. I've been using it for at least 10 months exactly the way I want. One breaking change to the format happened during this time and I was able to adjust to that change.

Now, today, it looks like another breaking change has happened and I'm curious to see if it's my code or if it's youtube's embed syntax.

So here's a list of youtube video id's from the Veritasium channel:

2KZb2_vcNTg,Ux33-5k8cjg,vqDbMEdLiCs,myh94hpFmJY,Iuv6hY6zsd0,vWVZ6APXM4w,BD6h-wDj7bw,1Xp_imnO6WE,38gVZgE39K8,XAgXwUwQoPA,rAYW9n8i-C4

I dynamically construct an iframe with this baseline syntax:

<iframe id="myiframe" class="myplaylist" width="720" height="405" src="https://www.youtube.com/embed/videoseries?playlist= <comma separated list of video id's> &amp;controls=1?enablejsapi=1" frameborder="0" allowfullscreen=""></iframe>

So a functional iframe element created from these two pieces looks this:

<iframe id="myiframe" class="myplaylist" width="720" height="405" src="https://www.youtube.com/embed/videoseries?playlist=2KZb2_vcNTg,Ux33-5k8cjg,vqDbMEdLiCs,myh94hpFmJY,Iuv6hY6zsd0,vWVZ6APXM4w,BD6h-wDj7bw,1Xp_imnO6WE,38gVZgE39K8,XAgXwUwQoPA,rAYW9n8i-C4&amp;controls=1?enablejsapi=1" frameborder="0" allowfullscreen=""></iframe>

This won't work as a snippet, but as an html element, it should show a youtube player, and in the upper right show a counter and drop down list of the queued videos as though you were looking at a playlist on the youtube site.

For example (Except this one is demonstrating the new problem):

It would help if someone would try this completed tag, and see if the embedded player functions. I can't tell what's gone wrong.