Previously used png transparent photos now showing with solid white background

Others also experience this issue:

Discussion thread:

Image Transparency Issues - Apple Community

To ensure you download the full resolution images from iCloud:

Download iCloud photos and videos - Apple Support

says

On your iPhone, iPad, or iPod touch

1. Go to Settings > [your name].
2. Tap iCloud, then tap Photos.
3. Select Download and Keep Originals.

You might want to reach out to the Photos Support Team:

There is a "Get Support" link (online chat, phone, eMail etc)

at the bottom of their webpage (link below):

Photos Support Website

Photos - Official Apple Support

or for iCloud:

iCloud Support

You could also send feedback to those who work behind-the-scenes at Apple. They likely won't reply, yet your feedback will reach those who work on Photos:

Webform for feedback:

Feedback - Photos - Apple

Wondering if you use iCloud ... if so you may find the full-resolution photos with their transparency intact are available for download there, while low-res versions are still available on your iPad / iPhone ... may depend on your settings and available storage space.

Discussion thread on a similar situation (2022):

Transparent Images - Apple Community

Wondering whether you usually store the images with transparent backgrounds in the Files app.

Seems it could be the Photos app that may not open images that have transparent backgrounds, without changing them to white.

(On Mac it is the "Preview" app that preserves the transparency)

Re:  "Is there a way to restore their transparency without doing individual images trying to return to original?"

In this discussion thread:Transparent PNG files not working on iPho… - Apple Community

GameBlox says:

“You have to turn off the "Optimize" option at "iCloud Photo Library"

and

“The only way I can get the transparencies back is to send myself an email with the image, then iCloud finally shares the uncompressed version”.