For the density mixing schemes to work reliably, the charge density mixer must be aware of all quantities that affect the total energy during the self-consistency cycle. For a standard DFT functional, this is solely the charge density. In case of meta-GGAs, however, the total energy depends on the kinetic energy density as well.
In many cases the density mixing scheme works well enough without passing the kinetic energy density through the mixer, which is why LMIXTAU=.FALSE., per default. However, when the selfconsistency cycle fails to converge for one of the density-mixing algorithms (for instance, IALGO=38 or 48), one may set LMIXTAU=.TRUE. to have VASP pass the kinetic energy density through the mixer as well.