Hello!

The German astronomer Felix Schmeidler published two works in 1985.

"Messung der Lichtablenkung während der Sonnenfinsternis am 15. Februar 1961"
Abstract:
"During the total eclipse, a plate of the stars near the Sun was taken
on the Monte Cónero near Ancona in Italy, on which 12 stars were
measurable. Comparison with another plate of the same stars taken at
night showed strong distortion on the eclipse plate which was very
probably due to anomalous refraction. The differences of coordinates
between both plates were represented by a general secondorder
expression. The value L = 1°.98± 0°.46 was derived for the gravitational
deflection on the solar limb."
https://onlinelibrary.wiley.com/doi/...sna.2113060207

and

"Zur Interpretation der Messungen der Lichtablenkung am Sonnenrand"
Abstract:
"The agreement between radiastronomical measurements of the
gravitational deflection of light at the solar limb and the theory of
relativity is compatible with the exces found by most optical observers.
The optical excess is caused by stars so near to the Sun that
radioastronomical observations are impossible. Optical and
radioastronomical measurements both make it likely that there is an
additional term inversely proportional to a power higher than the first
of the distance from the Sun. Results found by BOUET indicate that this
additional term is of solar origin."

He proposed the following correction to the Einstein formula:

dr = 1.75"/r + 0.3"/r^2

What physical phenomenon can cause the larger light deflection?

Schmeidler mentioned Bout. Here is his work: http://adsabs.harvard.edu/full/1982SoPh...78..385B

Best Regards
Walter Orlov