SQL Server 2014

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CREATE TABLE t (	
	[FormID] [nvarchar](25),
	[Value] [int],
	[LogDate] [DATE]
)

INSERT INTO t ([FormID],[Value],[LogDate])
VALUES
('Form2', 6, CONVERT(date,'10/12/19')),
('Form2', 8, CONVERT(date,'10/13/19')),
('Form1', 16, CONVERT(date,'10/12/19')),
('Form1', 3, CONVERT(date,'10/12/19')),
('Form1', 3, CONVERT(date,'10/14/19')),
('Form1', 8, CONVERT(date,'10/15/19')),
('Form1', 12, CONVERT(date,'10/21/19'))
7 rows affected

SELECT t.*
FROM t
LEFT JOIN (SELECT FormID, MAX(LogDate) AS recent_reset 
FROM t
WHERE Value<6
GROUP BY FormID) AS reset_calendar
ON t.FormID = reset_calendar.FormID
WHERE t.LogDate > reset_calendar.recent_reset OR reset_calendar.recent_reset IS NULL


FormID
Value
LogDate


Form2
6
2019-10-12

Form2
8
2019-10-13

Form1
8
2019-10-15

Form1
12
2019-10-21


SELECT temp.*,
       ROW_NUMBER() OVER (PARTITION BY temp.FormID ORDER BY temp.LogDate) AS Sequence
FROM (
  SELECT t.*
  FROM t
  LEFT JOIN (
    SELECT FormID, MAX(LogDate) AS recent_reset 
    FROM t
    WHERE Value<6
    GROUP BY FormID) AS reset_calendar
  ON t.FormID = reset_calendar.FormID
  WHERE t.LogDate > reset_calendar.recent_reset OR reset_calendar.recent_reset IS NULL)temp


FormID
Value
LogDate
Sequence


Form1
8
2019-10-15
1

Form1
12
2019-10-21
2

Form2
6
2019-10-12
1

Form2
8
2019-10-13
2

FormID	Value	LogDate
Form2	6	2019-10-12
Form2	8	2019-10-13
Form1	8	2019-10-15
Form1	12	2019-10-21