Confirmation emails are sometimes missing custom fields

[nistune]

Joomla 3.9.15
VirtueMart 3.6.10

First noticed this issue back in January when running VirtueMart 3.6.2

Seems to happen with some particular customers (cannot repeat with my test customers, but it happens with a semi regular customer who just ordered in Jan and now last weekend)

1. They order a product and add to cart
https://www.nistune.com/index.php/nistune-shop/nistune-boards/type-3-board-detail

Then they
2. select the vehicle type from droplist (Custom Field Type: String, List=yes, name T3 Vehicle)
3. add in part numbers using the textinput (Custom field Type: textinput, name ECU part)

Then checkout and part number shows up correctly in the order history\

Problem is now the initial Confirmation email is missing the custom ID text field. If I change states and resend the confirmation email then it shows up correctly. This previously always worked in versions prior to 3.6. So there is some difficult to repeat glitch now

Example initial Confirmation email :

Product Name:
Type 3 board TL

Example resend Confirmation email

Product Name:
Type 3 board TL
T3 Vehicle: S13 180SX (SR20DET)
ECU part: 23710-50F05

So you can see the first email has empty fields. I've disabled my OnePagecheckout plugin (this also broke with 3.6 and CMSMart has not fixed it yet after reporting it back in November) but that did not fix anything.

Any other questions or things to try, let me know. For now I will have to check my order list manually before processing orders due to issues it has caused with my customers recently