Thank you for the question @mmcanulty!
This can be a bit tricky to parse out. I'm sorry if I wasn't clear in my last post, but this isn't an issue which has been ongoing for multiple years. There are two issues here, with similar symptom/behavior.
First issue (reported towards the end of 2020 and resolved in Revit 2021.1.3 and Revit 2022): Slowly refreshing material thumbnails while opening the material browser in Revit 2021
This issue was related to a problem in reading material thumbnails created when opening the material dialog (leading to heavy CPU usage as thumbnails are re-created each time the dialog was opened). This appears to be related to the Windows permissions as users found that running Revit with elevated permissions was a workaround for this behavior.
Second issue (which you reported in March 2022 and is currently being investigated): Slowly refreshing material thumbnails while opening the material browser in Revit 2021.1.3 or later
This issue affects the creation of material thumbnails for certain materials (with a combination of three conditions, listed in the article above) where the processing gets hung up leading to heavy CPU usage each time the materials dialog is opened.
Regarding the change in material appearance: This only affects the preview of the material in the material dialog. This does not change the appearance of materials within the model. I've added a note to the article to clarify this point.
Regarding the Asphalt material, this material shouldn’t be affected by the issue as the "Asphalt – Dark Grey" asset is an Opaque material, and this issue affects Layered materials (such as metallic paint, carbon fiber, granite…) to see if a material is using a layered asset, check the Material Asset Browser)

Here are some layered material assets:

Please let me know if you have any further questions on this.
Lance Coffey
Technical Support Specialist