This started happening with the 25.3.1 update
See: https://community.adobe.com/t5/illustrator/changing-same-stroke-color-of-multiple-objects-deletes-the-fill-color/m-p/12133696#M280760

Changing the stroke color or weight of selected objects with different gradient fills, deletes the fill. If objects with a normal or pattern fill are also selected, there is no problem. It only happens when only different gradient fills are selected.

The following configurations have been tested and show the same error:
MacBook Pro (2019), MacOS Catalina (10.15.7), AI 25.3
iMac (2016), MacOS Catalina (10.15.7), AI 25.3 + 25.3.1
Windows 10, AI 25.3.1

Work steps
- Multiple objects with different spot colors as outline are marked
- Changed line width of all objects together

Expectation:
- All lines should get the same thickness, but remaining information (spot color names, attributes, etc.) must be kept (worked until AI 25.2.3)

Result:
- Line color is deleted -> all attributes are lost (overprinting, etc.)

The following configurations have been tested and show the same error:
MacBook Pro (2019), MacOS Catalina (10.15.7), AI 25.3
iMac (2016), MacOS Catalina (10.15.7), AI 25.3 + 25.3.1
Windows 10, AI 25.3.1

Work steps
- Multiple objects with different spot colors as outline are marked
- Changed line width of all objects together

Expectation:
- All lines should get the same thickness, but remaining information (spot color names, attributes, etc.) must be kept (worked until AI 25.2.3)

Result:
- Line color is deleted -> all attributes are lost (overprinting, etc.)

When you select 2 different color strokes with the same weight it takes off the color or it sets the weight to zero.

It happens on both macOS and Windows 10.

this happens to me as well with global colors

When you select several objects (with different colors) and try to add a stroke or change the thickness, the fill changes to none.
Another problem is when restoring version 25.2 all settings are removed (Illustrator opens as if it was a clean install) - good that I had a backup.

System: Windows 10 Pro 20H2
Illustrator 25.3.1

Same issue here. Just noticed this today after updating to 25.3.1 on Mac 10.15.7

Short clip showing bug. Actually, you can trigger this bug by performing any stoke based change. i.e., cap or corner style, stroke alignment, miter value, etc.

https://1drv.ms/v/s!AsieUUYx_Wh4kAZTidvUvKTjDwtk?e=ZnG72m

Same issue, what happened for this behavior to change suddenly???

I am on a PC, Windows 10 Pro using Adobe Illustrator 25.3 (64 bit). I have attached an example. If you select all three objects at once and try to change the stoke weight it clears the color and fill of all 3. However if it's CMYK colors it works fine, it only happens if they are spot colors. I get the same results on Adobe Illustrator 25.3 installed on a Mac OS. Both systems also have Esko Data Exchange 21.03 plug-ins installed.

Does not happen for me.
Does this happen in every file?
What are your specs? Can you share the file?

I have objects stroked 1 pt but in different spot colors. I select "Same stroke weight" then change the stroke to 2 pt and it all clears, no fill, no stroke, and no weight to the stroke??

This is happening to me again today ... it's clearly a bug!

When I "select same fill color", I have multiple glitches. In one case, this function selects all the same fill color objects as expected, but then if I subsequently adjust their fill color via the cmyk color sliders, it causes all of the objects to suddenly have a white stroke. In another case, it selects items that are NOT all the same fill color. This is wreaking havoc on my workflow... any fixes?