Why is polonium tetrafluoride non-existent?
Before going into details, let me state Wikipedia:

$\ce{^{208}PoF6}$ was probably successfully synthesised via the same
reaction (passing fluorine gas over $\ce{^{208}Po}$) in 1960 where a
volatile polonium fluoride was produced, but it was not fully
characterized before it underwent radiolysis and decomposed to
polonium tetrafluoride.

Even attempts were made to dissolve polonium metal in hydrofluoric acid but no fluoride salts were isolated as a free salt as they radiolysed and decomposed. According to this 1961 paper:

Although no fluoride salts of $\ce{Po}$ have been isolated, $\ce{Po}$
is quite soluble in $\ce{HF}$. Solutions containing 2  curies of
$\ce{Po}$ per ml of $\ce{1 N HF}$ are reported to be unsaturated.
Attempts to prepare a volatile fluoride by reaction with $\ce{F2}$ gas or
with liquid $\ce{BF3}$ have been unsuccessful. However recent work with $\pu{1
 mg}$ of $\ce{^{208}Po}$ indicated the formation of a volatile $\ce{Po}$ compound,
which decomposed before it could be isolated.

Polonium tetrafluoride is very difficult to isolate as it is seen to form either a complex or oxidize/decompose radiolytically. Also, bringing analytical data on such compounds is hard as they are produced in minute amounts possible in the range of micrograms. Below is a text from this e-book 1

A white solid possibly polonium tetrafluoride is obtained by treating
polonium hydroxide/tetrachloride with aqueous dilute hydrofluoric
acid. Treating this solid suspension in sulfur dioxide yields a bluish
grey product (possibly $\ce{PoF2}$) which quickly reverts back to the
solid on standing owing to radiolytic oxidation. The solubility of
$\ce{Po(IV)}$ increases with the concentration of hydrofluoric acid
indicating a complex formation
There has been unsuccessful attempts to make volatile hexafluoride by
passing fluorine to $\ce{^{210}Po}$ but recently a fluoride has been
made with $\ce{^{208}Po}$ plated on platinum. This product appers to
be quite stable in vapor phase but on cooling, a nonvolatile compound
is formed possibly polonium tetrafluoride resulting from radioactive
decomposition. Analytical data are not recorded for any polonium
fluorides owing to difficulty of determining fluoride ion at microgram
level.

Reference

Advances in Inorganic Chemistry and Radiochemistry, Academic Press, 1962
https://www.osti.gov/servlets/purl/4367751/4367751.PDF