I honestly have no idea what is causing your problem. All I can suggest is you try updating the graphics card driver from the manufacturers site, not from Microsoft update.
Here with Edge.
Does this happen on other websites or is it just on the BT one?
If it does happen on other sites, try starting your computer in safe mode with networking and see if the problem is still there.
See link how to run safe mode.
If the problem has gone it would point to a problem on your computer, most likely a software issue that is preventing the page displaying correctly.
I see the same as @Alan_Caskie on two Windows 10 PCs (one with version 1703 build 15063.674 and the other 1709 OS Build 16299.19) with Chrome Version 61.0.3163.100 and 62.0.3202.94. On those same PCs I don't see the issue with Microsoft Edge 41.16299.15.0 or IE 11.15.16299.0 with the arrows displayed correctly on those browsers.
I did take a look at the source using the developer tools and comparing Chrome and IE. Both show the same so it seems to be something with the way Chrome interprets the CSS. I've not had a chance to dig further at this stage, but at least it shows @Alan_Caskie is not the only one seeing this.
This is certainly an odd one. Chrome on Win 10 works fine here.
Something on the particular PC in use?
Possibly to do with the PC, but that's at least three different PC.
I've since tried with all extensions disabled, using HTTP and HTTPS, using Incognito, AVG anti virus disabled, standing on my head with two pencils up my nose whilst shouting 'wibble', none of which have made any difference. Shame as I really had high hopes for the pencil / wibble solution.
Original Graphics driver, no further updates
Windows update ran automativally yesterday, still no arrows.