How to prevent square wave oscillation circuit from producing backwards sawtooth shapes in LTSpice4
When I see these behaviors, it's usually because of the solver choice, the automatic step size that LTSpice uses as a delta-time, or else something to do with the reltol option.

This circuit really is critical in the sense that there are very short LED pulses separated by a long period during which \$C_2\$ charges up. So LTSpice might incorrectly choose a minimum time step here and also this may compound with the relative tolerances, too.

So. The first thing I usually try with astable multivibrators like this is to set the UIC flag in the .TRAN to make sure that it doesn't attempt to find a quiescent point, instead. If that doesn't do it, then I set the minimum timestep to something painfully short and see if that gets the needed time resolution. And the final trick I try is to set RELTOL to a small value.

(Sometimes, it's about the solver itself. But usually not. You can change it in the options dialog box though.)

In this case, the minimum timestep does appear to clear things up. But it is very slow. It's a lot easier to just add this Spice line to your schematic:

.options reltol=0.0000001


That should help. (But feel free to combine that with a minimum timestep. I think \$100\:\mu\textrm{s}\$ would also be fine, though to be sure I'd go to \$20\:\mu\textrm{s}\$ if I left reltol at its default value.)