The App’s shown type and availability of pins is determined by device type in your App settings… Did you make any changes? Also, sliders are outputs (typically for PWM digital output pins) and analog pins are inputs… so normally they would not be available as a slider option in the first place.
EDIT - OK, I did some research on the Photon and realised that some pins can have dual uses… I created a “test project” on my app (v.2.14.0). All looks good.
What version is your App and do you have a Local Server? if so make sure it is also updated.
Meanwhile… you can of course just use the virtual pins to do what you want with much more diversity and control.
Something like this should work… you may have to also set your pinMode() in setup.
BLYNK_WRITE(V0) // Slider set for V0 and MIN-MAX range of 0-1023 (or whatever your digital pin range is)
int slideValue = param.asInt(); // Get the slider value value.
analogWrite(A4, slideValue); // Write PWM value to Photon's A4 pin
}