Initially reported here (linked to my post describing what the exact issue was after a lot of testing, as original post stated it is related to the World Event, which was not the case) about seven months ago:
The arrows don’t show up if you have exactly 9 or 10 weapons, but you can drag down with the mouse (presumably slide on mobile) as a workaround.
This will become more and more of an issue as more kingdoms reach this threshold of having exactly 9 or 10 where they will likely remain for a year or more. Most people don’t know you can drag-scroll, and consoles don’t have that luxury (or do they have an equivalent?), so they’d have to know those weapons are there and manually filter.
And more info here showing a secondary but where the arrows show when they shouldn’t (and don’t work while you are on the final row all sets of (4n) displayed objects, where n is > 2. Also still a thing. Eg., go to Blackhawk, filter “not brown, not yellow”, show all, have exactly 12 troops filtered. First click of scroll down changes display from troops 1-8 to 5-12, but the scroll down buttons are still visible. Clicking to scroll down again does nothing. There is no 13th troop to display here. Drag scroll will allow you to show troops 9-12 as the top row and an empty row at the bottom. Not as much of an issue since theres nothing being actively blocked here by having the extraneous arrows, but it might be related to the original issue so it is worth mentioning.