Observing the same behaviour for Go 1.8, Go 1.9.5 and Go 1.10.5, not something introduced in 1.13.

34180.go contains the code from the playground mentioned above. https://play.golang.org/p/PNBiGNussAs

$ go1.8 run 34180.go
2019/09/10 22:10:03 Parsed %VER-1.4 into 1 4
2019/09/10 22:10:03 Failed to parse %ER-1.4 . Reason: bad verb '%%' for integer
2019/09/10 22:10:03 Parsed VER-1.4 into 1 4
2019/09/10 22:10:03 Failed to parse ER-1.4 . Reason: input does not match format

The logic fails here:

if format[i] != '%' {

This assumes that a scanning problem is signaled when i points to a non % rune, which is not true.
During a line 2 call in the original example, advance function fails to match runes E and V returning -1. At this point state is i = 0, w = -1 and format[i] = '%'. This causes scanning to continue, failing later at integer scanning.

CC @smasher164 Investigation done.

Another failing case would be https://play.golang.org/p/itkheUx5t3O

package main

import (
	"fmt"
	"log"
)

func main() {
	var n int
	if _, err := fmt.Sscanf("123 %", "%d%%", &n); err != nil {
		log.Println(err)
	} else {
		log.Println("OK", n)
	}
}

Error returned too few operands for format '%%'.
Expected error to be input does not match format.

/cc @robpike @martisch

Change https://golang.org/cl/202879 mentions this issue: fmt: fix handling of %% verb in Scanf