Web App Downgrading Custom Image Symbology

[SamGatter]

I am currently experiencing an issue where using custom images for point symbology is being downgraded to a much lower resolution when viewing the data in a specific web app. 

When viewing the data in the source web map it looks like this - 

However when viewing it in the impacted web app it looks like this - 

It is working fine in another app that the data is used in.

It was working fine yesterday and no changes have been made in the meantime.

I have attached the image used for the symbology.

Any help is greatly appreciated, 

Thanks, 

Sam