There are 3 of us playing on a 1.8 server and we noticed that the player icons would not be present. As time went on, I noticed that some players' icons would appear normally (say as round dots off the edge of a map) and then as I transitioned into the map, my round dot moved to behind the map item's graphic.

I've also noticed this with the pointer at the edge of a map.

The above behaviors make me believe the icons are generated on a layer below the map graphics.

The only fix thus far has been to restart the server.

Things we've tried and/or noticed:
» Updated server to 1.8.1.pre from 1.8, no change.
» On server (fresh ubuntu 14.04 LTS 64 install), switched from open-jre to oracle's Java 7. No effect.
» On clients, both Win 7 64bits, have tried the generic whatever browsers install 32 bit java as well as the downloaded from oracle 64 bit latest build. No effect.
» Sometimes the green pointers are not all present.
» There is always at least 1 green pointer on a map. There are 3 of us on our map#15 which is where we started.
» Going to the Nether definitely hides icons. One user loves the Nether and says quitting his client and getting back in fixes his map. Today, he requested a restart, so that's not 100% fix.
» I have never had a client restart fix the hidden pointer problem for me.
» Sometimes both of the other players appear on my map(s) normally.
» This can affect one or more maps simultaneously.
» We have 15 full-sized maps in our world, with the highest # of map_111.
» If map#15 is broken, it is entirely possible a different map shows no signs of misbehaving. In our case, I don't care, as 15 is well covered with rails, etc. So I have been on the map above (#43) with it working knowing that 15 was broken.
» There is no extra information written to the server's log when the map goes bad.
» I have had a map stop working as soon as 30 minutes after a server restart.
» Maps have proven to work for as long as 2 hours after a server restart. (longer periods possible, just haven't been observed as yet)

I've got a couple screenshots at home. I'll update with those tonight. Server and clients are under my control, so I can try anything and also share the world, if desired.