When using the lasso tool to make selections, it makes sense to choose 'Polygonal lasso tool' and then hold the 'alt' key to change into freeform lasso as needed. Relasing the 'alt' key goes back to polygonal mode.
On Windows 7 and using Photoshop CC 2018, this was fine. However, moving to a Windows 10 machine with PS CC 2018 (and even with PS CC 2019) the lasso line disappears after a few seconds of holding the 'alt' key down. This makes detailed masking very tricky as you need to do it in very small parts.
I can only imagine this is some sort of OS related issue as on a W7 machine, it was all fine. It is only now we have moved our office over to W10 that we are getting this bug.
The machines are all high end: mine is an AMD Ryzen Threadripper 2990WX with 64GB RAM.