It's a very common problem for less reputed SD cards. It can be fixed by doing a full format on the SD card. (Not a quick format because it doesn't check for bad sectors or data redundancy errors). Don't forget to keep a backup before formatting.
Just wanted to add an answer for this that I found on Reddit, in case it disappears.
This issue seemed to be fixed for a few people by doing the following:
Go to settings -> Apps and Notifications -> see all apps -> three dot menu -> show system. Scroll to Download Manager. Clear cache, clear storage, force stop, disable, enable.
Of course, your download ...