Disable all MODs an try again.

This issue is caused by mods, disable them and see if that fixes your issue.

Thank you for your feedback.

I understand this bug is caused by mods, but that doesn't make it any less of a bug. For units that use mods in operations, especially ACRE and Task Force Radio are essential to good communication, the in game radio is not able to do half of what these mods can do. So you can't just expect these units to turn off all mods. Which means you either have to sacrifice some really awesome weapons, or find away to avoid the bug. The bug happens even when the bipod is not deployed and you are just laying down. So the best solution so far is to remember to move to the sitting position before reloading, but that has a lot of risks.

I really hope that the devs will take a second look at this because it's not enough to just ask people to disable mods in a game that's designed to be usable with or without mods installed. The bug only happens when using weapons from the Marksman DLC like the MAR-10, and only when prone, I'm sure that there is some way to fix it and I consider it pretty important since many thousands of ARMA 3 players use mods. I was going to post a new thread about it till I found this one, and I know a lot of players that have experienced this bug.

EDIT:: I found another report of this bug where they said it is being worked on in the Dev branch, which is great to hear.

I would like to contribute that this bug is also present on my system while mods are running. Although in my case the animation gets stuck if I reload while the prone and bipod undeployed. Reloading with the bipod deployed seems to function normally.

I will echo as well that the bug only seems to affect weapons in the marksman DLC

After further testing on the new patch, I was able to cycle over 30 magazine and reloads without the bug when the bipod is deployed. However, if you are prone and the bipod is not deployed it happens to me 100% of the time. I have no had one good reload while prone without the bipod deployed on the new patch. Also, if you are close enough to a vehicle, ladder or any other interaction like that where you move into or onto an object you can do so and fix the bug. If you don't have those nearby or someone who can kill and revive you, you just have to repawn.

I'm able to start the bug when I just go prone and reload a marksman rifle. Don't need a bipod

With respect; if all the other ArmA content works correctly with mods and this issue only appears with the Marksman DLC content then it is reasonable to conclude that this issue is caused by the Marksman DLC.